1. Field of the Invention
The present invention relates generally to pumps for pumping concrete and like fluid mixtures, and, more particularly, to a pump of the type having a cranked delivery tube which swings in and out of register with a plurality of pumping cylinders, with longitudinal pressure being applied to the swing tube to effectuate a seal between it and the discharge ports of the pumping cylinders.
2. Background Art
In the laying of concrete, various automatic pumping systems have been suggested for moving the concrete from a transport truck mixer or other source to the deposit site. Among other factors, the relatively high viscosity and extreme abrasiveness of the concrete slurries play a central role in the design of these systems.
One system which has come into common use is the swing tube-type concrete pump. In systems of this kind, there is typically an open-topped hopper which receives the fluid concrete. A plurality of pumping cylinders (typically, two) are connected to the hopper so that their intake/discharge openings are in communication with the concrete in the hopper, usually near the bottom thereof. An S-shaped swing tube is mounted within the hopper so that it is pivotable about an axis provided by its discharge end, which is supported by a bearing in a wall of the hopper. The swing tube is connected to an external drive arm, which swings the tube back and forth so that its intake end moves into and out of register with the intake/discharge openings of the pumping cylinders. These cylinders operate 180.degree. out of phase, so that one cylinder is drawing a charge of concrete from the hopper while the other is discharging its charge under pressure. The action of the swing arm is timed so that its intake opening is moved into register with the intake/discharge opening of each pumping cylinder as this discharges its charge of concrete, thus clearing the opening of the other cylinder so that this latter can draw its charge from the hopper.
While the merits of the swing tube-type concrete pump have rendered its use widespread within the industry, such machines have not been without their inefficiencies and difficulties in operation. For example, it is essential that a relatively fluid-tight joint be achieved where the intake opening of the swing tube mates with the intake/discharge ports of the pumping cylinders, in order to ensure that the proper concrete mix is delivered from the hopper to the delivery line and the deposit site. If there is any significant leakage here, this may cause the liquid (i.e., water) within the concrete to be forced from the pumped concrete back into the hopper, being that the pumping pressures are frequently very high. Such a loss of moisture changes the mix or slump of the concrete as delivered, and under severe leakage conditions, the moisture loss may be sufficient to result in a dry pack within the pumping line, which completely freezes and closes the pumping line. If this occurs, the system must be shut down so that the line can be cleared. This shutdown is relatively costly, not only in terms of the efforts needed to perform the maintenance on the pumping line, but also due to the delays in the construction activities which are supported by the machine.
Although attempts have been made to eliminate such leakage by essentially "preloading" the intake opening of the swing tube against the openings of the pumping cylinders, this has usually resulted in excessive wear of the mating surfaces of these parts, largely due to the extreme abrasiveness of the concrete mix.
A number of approaches have been suggested in the past with respect to overcoming these problems. For example, U.S. Pat. No. 4,198,193 (Westerlund et al.) shows an arrangement in which a sealing assembly is fitted around the intake end of the swing tube, this having an annular recess which receives a wear ring. A chamber behind the wear ring is filled with constant hydraulic pressure so as to force the wear ring into firm sliding engagement with a wear plate around the openings of the pumping cylinders. Of course, this continuous pressure between the ring and plate results in excessive wear of these components, and the patentees in this case simply provide that the wear ring is a disposable item to be replaced as needed, which necessitates undesirable maintenance and downtime in a field environment.
U.S. Pat. No. 3,963,385 (Caban) also shows another arrangement wherein a constant sealing pressure is maintained between the receiving end of the swing tube and the ports of the pumping cylinders. In this case, this is done by applying the sealing pressure at the bearing where the discharge end of the swing tube passes out of the hopper. A pressure ring (apparently threaded) is tightened against the sealed bearing which extends around the end of the swing tube, so as to force the other end of this against the cylinder ports. The patentee states that if additional pressure is desired, this can be supplied by also applying hydraulic force against the back side of the bearing.
Some systems have been proposed which exhibit a higher level of sophistication in that the application of pressure to sealingly engage the swing tube and the cylinder ports is timed so that this is relieved at the end of the discharge stroke of each cylinder, with the result that the intake end of the swing tube is then freed to move between the openings without being forced against them.
U.S. Pat. No. 4,569,642 (Dwyer) shows an arrangement of this type. A bracket around the receiving end of the swing tube extends out of the top of the hopper, with a hydraulic cylinder being mounted on the end of this to push against an outside wall of the hopper so that the force which is generated thereby is transmitted through the bracket to the end of the swing tube to achieve the sealing force; alternatively, the hydraulic ram is mounted to the end of the shaft of the pivot mounting of the swing tube so that the force is transmitted through this. In either case, the clamping action which is provided by the hydraulic ram is actuated when the intake end of the swing tube is positioned around one of the cylinder ports to receive a charge of concrete therefrom, and then is released so that the tube can be swung into register with the next cylinder. Whatever the effectiveness of this system, it represents a relatively cumbersome structural arrangement which necessitates the use of dedicated and expensive subassemblies.
U.S. Pat. No. 4,614,483 (Huldelmaier) shows another "timed" system for providing a seal between the pumping cylinders and the swing tube, this differing from the systems described above in that the seal is achieved by extending a sleeve out of the discharge port of the cylinder and into the mouth of the intake end of the swing tube, instead of by forcing the swing tube against the cylinder port. The sleeve member which is extended either by reaction of threaded portions of the member, or by the application of hydraulic pressure to annular chambers in front of and/or behind the sealing sleeve. The sleeve member is then withdrawn at the end of the discharge stroke of the cylinder so that the swing tube can be moved to the next port. Again, this would appear to be a relatively complicated and expensive arrangement, and one of doubtful reliability in the field.
U.S. Pat. No. 3,982,857 (Schlecht) shows a somewhat tidier arrangement in which the swing tube (which is downwardly extending in this case) is supported by a bearing in a transverse beam. In the lower end of this beam (i.e., toward the pumping cylinder ports), there is an annular hydraulic piston which abuts a shoulder around the exterior of the swing tube. An annular chamber is formed between the bearing body and the annular piston. When the inlet opening of the swing tube is positioned around a cylinder port, pressure is admitted to the cylinder space so that the piston, abutting against the shoulder, presses the intake opening of the tube against the wall around the port. Then, when the swing tube is to be moved to the other opening, the hydraulic pressure is relieved. A labyrinth assembly is mounted between the bin and the hydraulic piston so as to prevent contamination of the latter.
While the sealing assembly which is disclosed in the Schlecht patent may be generally simpler and neater in its construction than some of the other devices which have been described above, and also may be somewhat more efficient in applying a sealing force between the intake of the swing tube and the openings of the pumping cylinders, this device is nevertheless not without its drawbacks. For example, the arrangement requires the complexity of having separate piston and bearing assemblies, and the hydraulic piston assembly itself is relatively complex and must be manufactured to close tolerances. Apart from adding to the expense of manufacture, such complexity would also tend to reduce durability in operation. Furthermore, the arrangement requires the use of a dedicated hydraulic line for supplying pressure to the piston assembly.
Accordingly, there exists a need for a concrete pump of the swing tube-type which is provided with a simplified and effective system for pressing the intake end of the swing tube into engagement with receiving areas about the intake/discharge ports of the multiple pumping cylinders of the pump so as to achieve an effective fluid-tight seal therewith, and which releases such pressure after the completion of the discharge stroke so that the tube can be swung into register with the port of the next cylinder. Furthermore, the need exists for a pump having such a system which makes efficient use of existing structures which are required by the nature of the swing tube-type design, and which minimizes the need for dedicated and complex assemblies to achieve the desired sealing pressure. Still further, there exists a need for a pump having such a system which is both reliable in the field and durable over extended periods of use.